The Radiocarbon Clock
It Dates Once-Living Remains. Or Does It?
ALL the foregoing clocks run so slowly that they are of little or no use in studying archaeological problems. Something much faster is needed to match the time scale of human history. This need has been met by the radiocarbon clock.
Carbon 14, a radioactive isotope of ordinary carbon 12, was first found in atom-smashing experiments in a cyclotron. Then it was found also in the earth’s atmosphere. It emits weak beta rays, which can be counted by a suitable instrument. Carbon 14 has a half-life of only 5,700 years, which is suitable for dating things associated with man’s early history.
The other radioactive elements we have discussed have lives that are long compared to the earth’s age, so they have existed since earth’s creation down to the present day. But radiocarbon has such a short life, relative to the earth’s age, that it can still be here only if it has been continually produced in some way. That way is the bombardment of the atmosphere by cosmic rays, which convert nitrogen atoms into radioactive carbon.
This carbon, in the form of carbon dioxide, is used by plants in the process of photosynthesis and is converted into all kinds of organic compounds in living cells. Animals and, yes, we humans, eat the plant tissues, so everything that lives comes to contain radiocarbon in the same proportion as it is found in the air. As long as anything lives, the radiocarbon in it, which decays, is replenished by fresh intake. But when a tree or an animal dies, the supply of fresh radiocarbon is cut off, and the radiocarbon level in it begins to drop. If a piece of wood charcoal or an animal bone is preserved for 5,700 years, it will contain only half as much radiocarbon as it had when alive. So, in principle, if we measure the proportion of carbon 14 remaining in something that once was alive, we can tell how long it has been dead.
The radiocarbon method can be applied to a wide variety of things of organic origin. Many thousands of samples have been dated by it. Their fascinating diversity is suggested by just a few examples:
Wood from the funerary ship found in the tomb of Pharaoh Seostris III was dated at 1670 B.C.E.
Heartwood from a giant redwood in California, which had 2,905 annual rings when it was cut down in 1874, was dated at 760 B.C.E.
Linen wrappings from the Dead Sea Scrolls, dated to the first or second century B.C.E. by the style of handwriting, were measured by the radiocarbon content to be 1,900 years old.
A piece of wood found on Mt. Ararat, and considered by some to be possibly from Noah’s ark, proved to date only from 700 C.E.—old wood, indeed, but not nearly old enough to predate the Flood.
Woven rope sandals dug out of volcanic pumice in an Oregon cave showed an age of 9,000 years.
Flesh from a baby mammoth, frozen in Siberian muck for thousands of years, was found to be 40,000 years old.
How reliable are these dates?
Errors in the Radiocarbon Clock
The radiocarbon clock looked very simple and straightforward when it was first demonstrated, but it is now known to be prone to many kinds of error. After some 20 years’ use of the method, a conference on radiocarbon chronology and other related methods of dating was held in Uppsala, Sweden, in 1969. The discussions there between chemists who practice the method and archaeologists and geologists who use the results brought to light a dozen flaws that might invalidate the dates. In the 17 years since then, little has been accomplished to remedy these shortcomings.
One nagging problem has always been to ensure that the sample tested has not been contaminated, either with modern (live) carbon or with ancient (dead) carbon. A bit of wood, for example, from the heart of an old tree might contain live sap. Or if that has been extracted with an organic solvent (made from dead petroleum), a trace of the solvent might be left in the portion analyzed. Old buried charcoal might be penetrated by rootlets from living plants. Or it might be contaminated with much older bitumen, difficult to remove. Live shellfish have been found with carbonate from minerals long buried or from seawater upwelling from the deep ocean where it had been for thousands of years. Such things can make a specimen appear either older or younger than it really is.
The most serious fault in radiocarbon-dating theory is in the assumption that the level of carbon 14 in the atmosphere has always been the same as it is now. That level depends, in the first instance, on the rate at which it is produced by cosmic rays. Cosmic rays vary greatly in intensity at times, being largely affected by changes in the earth’s magnetic field. Magnetic storms on the sun sometimes increase the cosmic rays a thousandfold for a few hours. The earth’s magnetic field has been both stronger and weaker in past millenniums. And since the explosion of nuclear bombs, the worldwide level of carbon 14 has increased substantially.
On the other hand, the proportion is affected by the quantity of stable carbon in the air. Great volcanic eruptions add measurably to the stable carbon-dioxide reservoir, thus diluting the radiocarbon. In the past century, man’s burning of fossil fuels, especially coal and oil, at an unprecedented rate has permanently increased the quantity of atmospheric carbon dioxide. (More details on these and other uncertainties in the carbon-14 clock were given in the April 8, 1972, issue of Awake!)
Dendrochronology—Dating by the Growth Rings of Trees
Faced with all these fundamental weaknesses, the radiocarbon people have turned to standardizing their dates with the help of wood samples dated by counting tree rings, notably those of bristlecone pines, which live hundreds and even thousands of years in the southwestern United States. This field of study is called dendrochronology.
So the radiocarbon clock is no longer regarded as yielding an absolute chronology but one which measures only relative dates. To get the true date, the radiocarbon date has to be corrected by the tree-ring chronology. Accordingly, the result of a measurement of radiocarbon is referred to as a “radiocarbon date.” By referring this to a calibration curve based on tree rings, the absolute date is inferred.
This is sound for as far back as the bristlecone ring count is reliable. The problem now comes up that the oldest living tree whose age is known goes back only to 800 C.E. In order to extend the scale, scientists try to match overlapping patterns of thin and thick rings in pieces of dead wood found lying nearby. By patching together 17 remnants of fallen trees, they claim to go back over 7,000 years.
But the tree-ring standard does not stand alone either. Sometimes they are not sure just where to put one of the dead pieces, so what do they do? They ask for a radiocarbon measurement on it and use that as a guide in fitting it in. It reminds one of two lame men with only one crutch between them, who take turns using it, one leaning for a while on his partner, then helping to hold him up.
One must wonder at the miraculous preservation of loose bits of wood lying so long in the open. It would seem they might have been washed away by heavy rainfall or picked up by passersby for firewood or some other use. What has prevented rot or insect attack? It is credible that a living tree might withstand the ravages of time and weather, an occasional one surviving for a thousand years or more. But dead wood? For six thousand years? It strains credibility. Yet this is what the older radiocarbon dates are based on.
Nevertheless, the radiocarbon experts and the dendrochronologists have managed to put aside such doubts and smooth over the gaps and inconsistencies, and both feel satisfied with their compromise. But how about their customers, the archaeologists? They are not always happy with the dates they get back on the samples they send in. One expressed himself this way at the Uppsala conference:
“If a carbon-14 date supports our theories, we put it in the main text. If it does not entirely contradict them, we put it in a footnote. And if it is completely ‘out of date,’ we just drop it.”
Some of them still feel that way. One wrote recently concerning a radiocarbon date that was supposed to mark the earliest domestication of animals:
“Archeologists [are coming] to have second thoughts about the immediate usefulness of radiocarbon age determinations simply because they come out of ‘scientific’ laboratories. The more that confusion mounts in regard to which method, which laboratory, which half-life value, and which calibration is most reliable, the less we archeologists will feel slavishly bound to accept any ‘date’ offered to us without question.”
The radiochemist who had supplied the date retorted: “We prefer to deal with facts based on sound measurements—not with fashionable nor emotional archeology.”
If scientists disagree so sharply about the validity of these dates reaching back into man’s antiquity, is it not understandable that laymen might be skeptical about news reports based on scientific “authority,” such as those quoted at the head of this series of articles?
Direct Counting of Carbon 14
A recent development in radiocarbon dating is a method for counting not just the beta rays from the atoms that decay but all the carbon-14 atoms in a small sample. This is particularly useful in dating very old specimens in which only a tiny fraction of the carbon 14 is left. Out of a million carbon-14 atoms, only one, on the average, will decay every three days. This makes it quite tedious, when measuring old samples, to accumulate enough counts to distinguish the radioactivity from the cosmic-ray background.
But if we can count all the carbon-14 atoms now, without waiting for them to decay, we can gain a millionfold in sensitivity. This is accomplished by bending a beam of positively charged carbon atoms in a magnetic field to separate the carbon 14 from the carbon 12. The lighter carbon 12 is forced into a tighter circle, and the heavier carbon 14 is admitted through a slit into a counter.
This method, although more complicated and more expensive than the beta-ray-counting method, has the advantage that the amount of material needed for a test is a thousand times less. It opens up the possibility of dating rare ancient manuscripts and other artifacts from which a sample of several grams that would be destroyed in testing just cannot be had. Now such articles can be dated with just milligrams of sample.
One suggested application of this would be to date the Shroud of Turin, which some believe Jesus’ body was wrapped in for burial. If radiocarbon dating was to show that the cloth is not that old, it would confirm the suspicions of doubters that the shroud is a hoax. Until now, the archbishop of Turin has refused to donate a sample for dating because it would take too large a piece. But with the new method, one square centimeter would be enough to determine whether the material dates from the time of Christ or only from the Middle Ages.
In any event, attempts to extend the time range have little significance as long as the greater problems remain unsolved. The older the sample is, the more difficult it is to ensure the complete absence of slight traces of younger carbon. And the farther we try to go beyond the few thousand years for which we have a reliable calibration, the less we know about the atmospheric level of carbon 14 in those ancient times.
Several other methods have been studied for dating events in the past. Some of these are related indirectly to radioactivity, such as the measurement of fission tracks and radioactive halos. Some involve other processes, such as the deposition of varves (layers of sediment) by streams flowing from a glacier and the hydration of obsidian artifacts.
The racemization of amino acids is another dating method used. But what does “racemization” mean?
Amino acids belong to the group of carbon compounds that have four different groups of atoms attached to a central carbon atom. The tetrahedral arrangement of the groups makes the molecule asymmetrical as a whole. Such molecules exist in two forms. Although chemically identical, one is physically the mirror image of the other. A simple illustration of this is a pair of gloves. They have the same size and shape, but one fits only your right hand, the other only your left.
A solution of one form of such a compound twists a beam of polarized light to the left; the other kind rotates it to the right. When a chemist synthesizes an amino acid from simpler compounds, he gets equal amounts of both forms. Each form cancels out the effect of the other on polarized light. This is called a racemic mixture, when both left-handed and right-handed amino acids are equally present in the mixture.
When amino-acid compounds are formed in living plants or animals, they come in only one form, usually the left-handed, or l- (for levo-) form. If such a compound is heated, the thermal agitation of the molecules turns some of them inside out, changing the left-handed form to the right-handed (the dextro form). This change is called racemization. Continued long enough, it produces equal amounts of the l- and d-forms. It is of special interest because it relates to living things, as does radiocarbon dating.
At lower temperatures, racemization goes at a slower pace. How much slower depends on the energy it takes to invert the molecule. It follows a well-known chemical law, known as the Arrhenius equation. If the amino acid is cooled more and more, the reaction goes slower and slower until, at ordinary temperatures, we cannot see it changing at all. But we can still use the equation to calculate how fast it is changing. It turns out that it would take tens of thousands of years for a typical amino acid to approach the racemized state, when both left-handed and right-handed forms of the amino acids are present in equal quantities.
The idea for dating by this method is this: If a bone, for example, is buried and left undisturbed, the aspartic acid (a crystallized amino acid) in the bone is slowly racemized. We dig up the bone a long time later, extract and purify the remaining aspartic acid, and compare its degree of polarization with that of pure l-aspartic acid. Thus we can estimate how long ago the bone was part of a living creature.
The decay curve is similar to that of a radioactive element. Each amino acid has its own characteristic rate of decay, just as uranium decays slower than potassium. However, note this important difference: Radioactive rates are unaffected by temperature, whereas racemization, being a chemical reaction, is markedly dependent on temperature.
Some of the most highly publicized applications of the racemization method have been to human skeletal remains found along the coast of California. One, called the Del Mar man, was dated by this method at 48,000 years. Another, the skeleton of a female found in an excavation near Sunnyvale, appeared to be even older, a startling 70,000 years! These ages created quite a stir not only in the public press but especially among paleontologists, because no one had believed that man was in North America that long ago. Speculation arose that man could have wandered across the Bering Strait from Asia as much as a hundred thousand years ago. But how certain were the dates turned out by this novel method?
To answer this, tests were made by a radioactive method involving intermediate decay products between uranium and lead that have half-lives suitable for this range. This gave ages of 11,000 years for the Del Mar skeleton and only 8,000 or 9,000 for the Sunnyvale. Something was wrong.
The big uncertainty in racemization ages is the unknown thermal history of the specimen. As mentioned above, the rate of racemization is extremely sensitive to temperature. If the temperature goes up by 25 degrees Fahrenheit (14° C), the reaction goes ten times as fast. How could anyone know what temperatures the bones could have been exposed to so many years in the past? How many summers might they have lain bare under a hot California sun? Or might they even have been in a campfire or a forest fire? Besides the temperature, other factors have been found to affect the rate greatly, such as the pH (degree of acidity). One report says: “Amino acids in sediments show an initial rate of racemization almost an order of magnitude (tenfold) faster than the rate observed for free amino acids at a comparable pH and temperature.”
Even that is not the end of the story. One of the Sunnyvale bones was tested for radiocarbon, both by the counting of beta particles from decaying atoms and by the newer atom-counting method. These gave roughly concordant values. The average was only 4,400 years!
What can we believe? Obviously some of the answers are terribly wrong. Should we put more confidence in the radiocarbon date, since there is longer experience in using it? But even with it, different samples from the same bone varied from 3,600 to 4,800 years. Perhaps we should just admit, in the words of the scientist quoted previously, “Maybe all of them are wrong.”
[Blurb on page 23]
The radiocarbon clock is now known to be prone to many kinds of error
[Box on page 22]
Just this year Science News, under the title “New Dates for ‘Early’ Tools,” reported:
“Four bone artifacts thought to provide evidence for human occupation of North America approximately 30,000 years ago are, at most, only about 3,000 years old, report archaeologist D. Earl Nelson of Simon Fraser University in British Columbia and his colleagues in the May 9 SCIENCE. . . .
“The difference in age estimates between the two types of carbon samples from the same bone is, to say the least, significant. For example, a ‘flesher’ used to remove flesh from animal skins was first given a radiocarbon age of 27,000 years old. That age has now been revised to about 1,350 years old.”—May 10, 1986.
[Diagram on page 24]
(For fully formatted text, see publication)
The amount of carbon 14 (or racemized aspartic acid) varies with external conditions
[Diagram on page 26]
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COOH C NH2 H CH2COOH
HOOC C H2N H HOOCH2C